SQL：获取每个分组中的最新条目以及每个分组中有多少条目

Question

Say I have the following table:

some_table
id  relid  upid  downid  sent                 text
1   1      1     1       2014-09-13 12:23:11  sometext bla
2   1      1     2       2014-10-08 14:15:09  othertext
3   3      2     4       2014-10-08 20:34:57  foo
4   2      1     1       2014-10-09 04:14:15  bla
5   1      1     1       2014-10-15 17:24:15  more text

So far, the following command

    SELECT
        p1.relid AS relid,
        p1.downid AS downid,
        p1.sent AS last,
        p1.`text` AS `text`
    FROM
        some_table AS p1
    INNER JOIN (
        SELECT
            MAX(sent) AS MaxDate,
            downid,
            relid
        FROM 
            some_table
        WHERE
            upid='1'
        GROUP BY
            downid,
            relid
    ) AS p2
    ON
        p1.downid = p2.downid AND
        p1.relid = p2.relid AND
        p1.sent = p2.MaxDate
    WHERE
        p1.upid='1'
    ORDER BY
        `last` DESC

gives me the follwing output

relid  downid  last                 text
1      1       2014-10-15 17:24:15  more text
2      1       2014-10-09 04:14:15  bla
1      2       2014-10-08 14:15:09  othertext

as expected. (btw, I got that solution from elsewhere on stackoverflow) However, I'd like to get something output that looks like this:

relid  downid  last                 text       count
1      1       2014-10-15 17:24:15  more text  2
2      1       2014-10-09 04:14:15  bla        1
1      2       2014-10-08 14:15:09  othertext  1

where "count" is the number of rows that were grouped by "GROUP BY".

Is that at all possible, and if so, how?

An alternative would be to have two tables

sometable1
id  relid  upid  downid  last
1   1      1     1       2014-10-15 17:24:15
2   1      1     2       2014-10-08 14:15:09
3   3      2     4       2014-10-08 20:34:57
4   2      1     1       2014-10-09 04:14:15
sometable2
id  cid  sent                 text
1   1    2014-09-13 12:23:11  sometext bla
2   2    2014-10-08 14:15:09  othertext
3   3    2014-10-08 20:34:57  foo
4   4    2014-10-09 04:14:15  bla
5   1    2014-10-15 17:24:15  more text

and then do something like

    SELECT
        sometable1.id AS cid1,
        sometable1.relid AS relid,
        sometable1.downid AS downid,
        sometable1.last AS last,
        sometable2.text AS `text`,
        COUNT(SELECT
            id
        FROM
            sometable2
        WHERE
            sometable2.cid=cid1) AS `count`
    FROM
        sometable1
    LEFT JOIN
        sometable2
    ON
        sometable2.cid=sometable1.id AND
        sometable2.sent=sometable1.last
    ORDER BY
        last DESC

except that apparently produces a syntax error

Answer 1

You can just an an additional column to your subquery ( p2 ) as something like COUNT(*) AS CountOfRecords , then you can select this in the outer query:

SELECT
    p1.relid AS relid,
    p1.downid AS downid,
    p1.sent AS sent,
    p1.`text` AS `text`,
    p2.CountOfRecords
FROM
    some_table AS p1
INNER JOIN (
    SELECT
        MAX(sent) AS MaxDate,
        COUNT(*) AS CountOfRecords,
        downid,
        relid
    FROM 
        some_table
    WHERE
        upid='1'
    GROUP BY
        downid,
        relid
) AS p2
ON
    p1.downid = p2.downid AND
    p1.relid = p2.relid AND
    p1.sent = p2.MaxDate
WHERE
    p1.upid='1'
ORDER BY
    `sent DESC